1. Field of the Invention
The present invention relates to capacitance measuring circuits and in particular to a capacitance measuring circuit that produces an electrical output signal proportional to capacitance changes and to a capacitive field sensor that detects a field disturbance in an emitted field.
2. Description of Prior Art
Capacitance measuring circuits are used in many types of transducers in which a change in capacitance results from a change in the gap or variation in the area of the capacitor plates or a change in the size and/or properties of the dielectric between the plates. Some exemplary types of transducers include weight scales, thickness gauges, liquid level sensors, linear and rotary position sensors, accelerometers, and pressure transducers.
The prior art is replete with circuit arrangements for measuring an unknown capacitance. While an unknown capacitance C1 may be measured directly, improved sensitivity is usually obtained by comparing it to a second capacitance, C2. Such methods for producing a differential signal have been extensively used in prior art. As described in a book, xe2x80x9cCapacitive Sensorsxe2x80x9d by Larry K. Baxter, IEEE Press, 1997, the differential methods produce an output signal which is quantified by either V(C1xe2x88x92C2)/(C1+C2), V(C1xe2x88x92C2)/Cf, or VC1/C2, where V is an excitation voltage and Cf is a circuit feedback capacitance. Generally these methods employ an amplification stage, followed by a detection stage that translates an AC signal produced by a capacitor bridge into a DC signal related to the unknown capacitance. This DC signal is typically amplified further to produce a useful signal level.
If this multiplicity of stages is eliminated such that a first stage produces an adequate DC signal level, noise performance can be greatly improved. Because the output signal of the first stage determines not only the sensitivity, but also the noise performance of the overall system, it is important for this signal to have its largest possible value. Moreover, as C1 and C2 increase in magnitude while at the same time the quantity (C1xe2x88x92C2) decreases in an application, an adequate first stage response becomes more and more desirable. Obviously one way to increase the amplitude of the output signal of the first stage is to increase the excitation voltage V. There are, however, practical limits to how much the excitation voltage can be increased because of considerations arising from other circuit components, including the power supply. Similarly, the magnitude of the value V(C1xe2x88x92C2)/Cf can be increased by decreasing Cf, but circuit stability considerations again limit the extent to which it can be reduced.
Three plate differential capacitors, comprising two fixed and one central movable plate, are frequently used in transducers. Since the quantity V(C1xe2x88x92C2)/(C1+C2) is directly proportional to the displacement of the central plate, circuits based on measuring this displacement are particularly popular for use in accelerometers and pressure sensors. Many such circuits, however, are configured such that the movable plate is electrically active, necessitating electrical isolation from its mechanical support. Also, to improve performance, an electrically active shield is often used to guard the movable plate. These circuit requirements increase the mechanical complexity of three plate systems. Moreover, when capacitance is large and the capacitance change to be measured is small, the magnitude of V(C1xe2x88x92C2)/(C1+C2) tends to be small because of the large denominator. In addition, any stray capacitance that may be present becomes an additive term in the denominator, leading to a further reduction in first stage response.
In U.S. Pat. No. 4,193,063, Hitt and Mergner disclose a circuit for producing an output signal that is linearly related to the displacement of the grounded central plate of a differential capacitor. The capacitors are charged in parallel from a constant current source for a fixed period of time and discharged through separate circuits for another period of time. As the capacitors discharge, an integrating differential amplifier produces an output signal related to the difference in area between the two discharge curves. This circuit uses an amplifier with high common mode rejection and also uses a charge period that is sufficiently long to inject detectable levels of charge.
In U.S. Pat. No. 4,287,471 Ko and Hung disclose a circuit wherein differential capacitors are energized by a square wave signal. The variation in capacitance is then detected by a diode bridge and amplified to produce an output signal indicative of the centering of a grounded metal strip. This circuit also uses an amplifier with high common mode rejection and, in addition, the excitation signal has a symmetrical waveform.
In U.S. Pat. No. 4,896,098 Haritonidis et al. disclose a circuit in which the central plate of a differential capacitor is driven by an AC generator. A capacitive voltage divider is formed with each of the stationary plates by means of two additional capacitors connected to ground. The output signal of each voltage divider drives a respective field effect transistor supplying current to the input terminal of a respective amplifier. The output signals from these two amplifiers are provided to a differential amplifier to produce a signal indicative of capacitance change. This circuit also uses an amplifier with high common mode rejection and a symmetrical excitation waveform.
Capacitance measuring circuits are used in a variety of applications, including proximity sensors, lamp dimmers, limit switches, and touch screens. All of these devices function by detecting capacitance changes caused by objects entering an emitted electrical field. While two plate capacitors can be used for this purpose, one plate circuits yield better detection range in distance. Typically, the capacitor for these applications comprises a single plate connected to a suitable circuit to emit a periodic electrical field that propagates out into space, the second plate of the capacitor being made up by the first plate""s environment of nearby objects and ground. When the position or the dielectric properties of proximate objects change, the capacitance measured at the first plate undergoes change. To detect this change, prior art includes methods based on sensing imbalance in capacitance bridges, shift in resonance frequency of tuned circuits, and timing changes in resistor capacitor (RC) circuits. Common shortcomings of these circuits include insensitivity to small changes in large capacitance and spurious response to the presence of moisture near the sensing plate. Additionally, when several sensors are used near each other, their performance may be degraded by cross talk among adjacent electric fields. Moreover, the continuous wave excitation used in the prior art has the potential for radio frequency interference.
To overcome some of these shortcomings, the prior art includes some proximity sensors that are claimed to function in the presence of moisture or other weakly conducting liquid media. Among these, are those which infer the value of an unknown capacitance utilizing short duration charge and/or discharge pulses.
One transducer of this type is disclosed in U.S. Pat. No. 5,730,165. This patent concerns a circuit for charging a sensing electrode and a switching element acting to remove that charge and transfer it to a charge detection circuit. The time duration of the charge and discharge steps can vary widely, with at least one of the steps being of a shorter duration than a characteristic conduction time for water. The teachings disclose that as the charge and/or discharge duration is decreased, apparent capacitance due to moisture also decreases. A duration of 100 nsec. or less is suggested as being optimal, for example, when detecting a human hand in the presence of water. The sensitivity of the disclosed circuit, however, depends on how many charge/discharge pulses are used while accumulating the charge transferred to the charge detector. As a result, gain of the circuit is severely bandwidth limited. Also, because the circuit is single ended, it does not provide the benefits that a differential configuration would.
Still other applications of capacitance measuring circuits include sensors for moisture content of wood, or ripeness of fruit, based on detecting capacitance changes due to variation of dielectric properties. These sensors are generally intended for the wide-ranging consumer markets and hence, are extremely price sensitive. The complexity of circuits disclosed in prior art, with requirements for high parts count and cost, makes them unattractive for these applications.
The present invention is embodied in a capacitance sampler having sensitivity that is directly proportional to sampling frequency and to differential capacitance, regardless of the size of each capacitance. This sensor has improved noise performance when compared to prior art capacitance sensors. The principle underlying the invention is that when two capacitors charged to voltages equal in magnitude but opposite in polarity are connected in parallel, they become discharged and any residual charge is a direct measure of the difference in their capacitance. The invention employs repetitive charge/discharge cycles, depending on the application. During each cycle, capacitance is sampled by fully charging the capacitors during a portion of the charge/discharge cycle, and discharging the capacitors during the remaining portion of the cycle. The residual charge can be measured either in a current mode or a voltage mode. Because this charge is acquired from the charging pulse, differential capacitance is sampled during the duration of the charging pulse.
One exemplary embodiment of the invention comprises a pulse generator which produces low duty cycle, complementary charging pulses. A capacitance sampler samples a differential capacitance to provide a signal to an output device operated in a current mode. The capacitance sampler includes an RC bridge, comprising at least one capacitor in one branch and at least one resistor in the other branch. Each resistor, capacitor junction of the bridge is connected to the pulse generator through a switching diode, such that each diode conducts current through the bridge during a short charging period and isolates the bridge from the pulse generator during a longer discharge period of the pulse repetition interval. The capacitor side of the bridge circuit is connected to a voltage reference, usually ground. The resistor side of the bridge is connected to an amplifier that is configured to measure the residual charge on the capacitor side of the bridge. In the preferred current mode embodiment, a current to voltage converter drains the residual charge at this junction and provides a voltage output signal given substantially by the quantity Vf(C1xe2x88x92C2)R, where V is an excitation voltage, f is a drive frequency, R is the resistance of a feedback resistor, and (C1xe2x88x92C2) is the unknown differential capacitance. To attenuate artifacts from bridge excitation, an optional low pass filter may be used to filter the output signal. In one embodiment of the invention, a single amplifier produces an output signal proportional to differential capacitance. Consequently, noise performance is greatly enhanced over the prior art capacitance measuring devices.